As the first batch of ex-fleet 1999 T-reg cars prepare to hit the used market, experts are predicting values will suffer because of the artificially older appearance of the letter prefix registration plates.
Furthermore, fleets can expect residual values for '51' and '02' cars to suffer as last year's record new car sales return to the used market in three years time.
Martin Ward, national research manager at CAP Network, said: 'X-registration cars are just over a year old now, but they look so dated compared to the new registrations, and it is making older cars devalue more quickly.'
The Society of Motor Manufacturers and Traders (SMMT) is predicting another bumper month for sales in March, although not quite as high as last year.
March sales have grown from 213,374 in 1998 - the last year of single August registrations - to 370,060 in the year of 99T cars.
In 2000, March sales reached 401,972 for W-registered cars, while last year it peaked at 408,024 for 01Y cars.
A spokesman for the SMMT said: 'It looks like this year will be another strong year for car registrations and we are currently predicting a total of about 2.3 million.
'As usual March and September will be the strongest months, and although we are unlikely to reach the levels of last year, there is much optimism among the manufacturers for new car sales.'
Manufacturers are already promoting retail offers for 02 vehicles, with Volvo offering free satellite navigation and a year's free insurance on S40, V40 and V70 models. But at CAP, Ward warned that the high volumes of new cars being sold now will hurt the residual values of these vehicles when they are de-fleeted in three years' time. He said: 'When there are a lot of similar cars hitting the market at the same time, it always detracts from their value.
'So there is a danger that with so many 51 and 02 registration cars being sold, when they go into the trade in three years, values will be weaker.'